The Food and Drug Administration on Tuesday approved a new Pfizer drug to treat a rare form of blood and bone marrow cancer that causes a buildup of unhealthy white blood cells. Pfizer’s Bosulif is a daily pill to treat chronic myeloid leukemia patients who carry a specific genetic variation. The disease is one of four types of the blood cancer, and accounts for about 15 percent of leukemia cases. The disease, which usually affects seniors, causes the bone marrow to produce unhealthy white blood cells that do not grow and die like normal cells. Instead, the cells buildup in the bone marrow and crowd out healthy cells that are needed to fight off infections.